This invention relates to a security element for an object of value, said element having a flat, transparent body with a front side and a back side between which there lies a center plane, a first region configured on the body and a second region configured on the body which encode a motif, said body having in the first region a ground element structure which conveys different color impressions from front and back sides in plan view of the body.
Security elements of value documents having periodic line gratings are known, for example from DE 102009012299 A1, DE 102009012300 A1 or DE 102009056933 A1. They can have color filter properties in the subwavelength region when the grating is so designed with regard to the grating profile that resonance effects occur in the visible wavelength region. Such color filter properties are known both for reflective and for transmissive subwavelength structures. Said structures have a strongly polarizing influence on the reflection or the transmission of an incident light ray. The color is relatively strongly dependent on angle in reflection or transmission of such subwavelength gratings. However, the color saturation is considerably weakened for said gratings when the incident light is unpolarized.
There is known a line grating having subwavelength structures which possesses angular-dependent, color-filtering properties. The line grating possesses a rectangular profile made of a dielectric material. The horizontal surfaces are overlaid with a high-refractive dielectric. Above this structure there is likewise located a dielectric material, with the refractive indices of the grating substrate and of the cover material preferably being identical. As a result there is formed an optically active structure consisting of two gratings made of the high-refractive material which are spaced by the height of the original rectangular profile. The grating ridges forming the line grating are made for example of ZnS. There can be produced therewith a color contrast in reflection, but in transmission a change of color tone for different angles is hardly perceptible. This structure is therefore only useful as a security feature in reflection and must be constructed on an absorbent ground for that purpose.
Two-dimensionally periodic hole structures are described in the scientific literature that have filter properties of the incident light in the subwavelength region. In this connection, reference is made to the following literature sources: T. W: Ebbesen et al., “Extraordinary optical transmission through sub-wavelength hole arrays”, Nature, 667-669 (1998); L. Martin-Moreno et al., “Theory of extraordinary optical transmission through subwavelength hole arrays”, Phys. Rev. Lett. 86(6), 1114-1117 (2001); W. L. Barnes et al., “Surface plasmon subwavelength optics”, Nature, Vol 424, Issue 6950, pp. 824-830 (2003); J. Bravo-Abad et al., “How light emerges from an illuminated array of subwavelength holes”, Nature Physics 1, 120-123 (2006); H. S. Lee et al., “Color filter based on a subwavelength patterned metal grating”, Opt. Express 15, 15457-15463 (2007); C.-P. Huang et al., “Dual effect of surface plasmons in light transmission through perforated metal films”, Phys. Rev. B 75, 245421 (2007). Such so-called hole arrays consist of thin opaque metal films.
DE 102007016394 A1 proposes employing such structures as security elements in value documents for authenticating genuineness.
Further, two-dimensionally periodic gratings are known that allow a color filtering in primary colors, red, green, blue, with high color saturation over a relatively great viewing-angle range. The publication B.-H. Cheong et al., “High angular tolerant color filter using subwavelength grating”, Appl. Phys. Lett. 94, 213104 (2009) describes a grating with cube-shaped elevations which has a pronounced band-pass filter characteristic. The elevations are made of amorphous silicon and located on a glass substrate. A replication method for such gratings is presented in the publications E.-H. Cho et al., “Two-dimensional photonic crystal color filter development”, Opt. Express 17, 8621-8629 (2009) and E.-H. Cho et al., “Nanoimprinted photonic crystal color filters for solar-powered reflective displays”, Opt. Express 18, 27712-27722 (2010).
The prior art further includes WO 2003/070482 A1, WO 2011/032665 A1, WO 2011/082761 A1, WO 2011/066992 A1, EP 2228671 A1. The publications R. J. Potton, “Reciprocity in optics”, Rep. Prog. Phys. 67, 717 (2004); R. Petit, “Electromagnetic theory of gratings”, Vol 22 of Topics in current physics, Springer-Verlag (1980), explain the principle of reciprocity in optics, in particular for gratings.
The grating structures from the prior art can be employed for security elements for protecting value documents, since they provide a color effect in plan view that is not reproducible by conventional copying methods. The angle dependency of the color effect is especially advantageous therefor.